Urban and suburban RF environments typically possess multiple reflection, scattering, and diffraction surfaces that can change the polarity of a transmitted signal and also create multiple images of the same signal displaced in time (multipath) at the receiver location. Within these environments, the horizontal and vertical components of the signal will often propagate along different paths, arriving at the receiver decorrelated in time and phase due to the varying coefficients of reflection, transmission, scattering, and diffraction present in the paths actually taken by the signal components. Note that the likely polarization angle of an antenna on a handset used in cellular communication systems to the local earth nadir is approximately 60.degree. towards horizontal (this may be readily verified by drawing a straight line between the mouth and ear of a typical human head and measuring the angle that the line makes with respect to the vertical). The resulting offset handset antenna propagates nearly equal amplitude horizontal and vertical signals subject to these varying effects of an urban/suburban RF environment. As a mobile phone user moves about in such an environment, the signal amplitude arriving at the antenna on the base station antenna the handset is communicating with will be a summation of random multiple signals in both the vertical and horizontal polarizations.
The summation of the random multiple signals results in a signal having a Rayleigh fading characterized by a rapidly changing amplitude. Because the signal arriving at the base station often has nearly identical average amplitude in the vertical and horizontal polarizations that are decorrelated in time and/or phase, the base station receiver may choose the polarization with the best signal level at a given time (selection diversity) and/or use diversity combining techniques to achieve a significant increase in the signal to noise ratio of the received signal.
Prior art base station antennas that may be used in a selection diversity or diversity combining system often use two separate linearly polarized antennas. This makes for a bulky and unwieldy arrangement because of the space required for each antenna and its associated hardware. U.S. Pat. No. 5,771,024, the contents of which are incorporated by reference, discloses a compact dual polarized split beam or bi-directional array. There is a need in the art, however, for a compact dual polarized boresight array.